A. Field of the Invention
This invention relates to the field of data communication and data-over-cable systems. More particularly, the invention relates to a method of broadcasting a message from a head-end or cable modem termination system of a data-over-cable system to a plurality of remotely located cable modems which happen to share a common channel in an upstream direction.
B. Description of Related Art
Cable modem systems for wide area network access, e.g., Internet access, are now being rolled out in selected metropolitan areas of the United States. Basically, these systems provide high-speed data communications over a cable television infrastructure. Background information related to cable modem systems in general is described in the Data-Over-Cable Service Interface Specifications (DOCSIS)xe2x80x94Radio Frequency Interface Specifications, Interim Draft, dated Jul. 24, 1998, issued by Cable Television Laboratories, Inc. This document, known to persons working in the art, is incorporated by reference herein in its entirety.
The basic overall architecture of a data over cable system is shown in FIG. 1. The system of FIG. 1 provides a mechanism by which a computer 10 connected to a backbone network 12 (either directly or indirectly by intermediate networks) may communicate with another computer 14 via a cable television infrastructure indicated generally by reference numeral 16. The cable television infrastructure 16 includes a distribution hub or xe2x80x9chead-endxe2x80x9d 18 that is connected to the backbone network 12 via a wide area network and a switch or router 20. A cable system head-end is a central location in the cable television network that is responsible for sending cable signals in the downstream direction. The head-end 18 modulates digital data into analog form and supplies analog signals to a fiber network 22, which is connected to a plurality of O/E nodes 24. The O/E nodes 24 convert optical signals in the fiber network 22 to electrical signals for transmission over a coax cable network 26 to a cable modem 28 at the customer""s location. The cable modem 28 demodulates the analog signals and extracts the digital data and supplies the data to the customer promises equipment 14, which, in a typical situation, is a general purpose computer in a home environment.
The head-end 18 includes a cable modem termination system or CMTS 30. This device provides a network side interface to a wide area network, indicated at 32, and an RF interface between the cable modem termination system and the cable network in both the downstream and upstream directions, indicated at 34 and 36. The term xe2x80x9cdownstreamxe2x80x9d, as used in the present document, refers to transmission in the direction from the head-end 18 or cable modem termination system 30 to the cable modem 28 at the customer premises. The term xe2x80x9cupstreamxe2x80x9d refers to transmission in the direction from the cable modem 28 at the customer premises to the cable modem termination system 30.
For transmission in the downstream direction, the CMTS 30 supplies data from the computer 10 to a modulation circuit MOD and to a combiner 38, where the data is combined with video signals for the cable television system. The combined signals are sent to a transmission module 40 where they are imparted onto the fiber network. In the receiving direction, data from the CPE 14 is received from the fiber network at a receive module 42, sent to a splitter and filter bank 44 and sent to a demodulation circuit DEMOD in the CMTS 30. The data is processed by a network termination unit 46, sent to the switch or router 20 and routed onto the WAN for transmission to the remote computer 10.
Many cable television networks provide only unidirectional cable systems, supporting only a xe2x80x9cdownstreamxe2x80x9d cable data path. A return data path via a telephone network (i.e., a xe2x80x9ctelephony returnxe2x80x9d), such as a public switched telephone network provided by ATandT, GTE, Sprint, MCI and others, is typically used for an xe2x80x9cupstreamxe2x80x9d data path. A cable television system with an upstream connection to a telephony network is called a xe2x80x9cdata-over-cable system with telephony return.xe2x80x9d Such a return system is indicated at 48 where the cable modem 28 is also shown connected to the public switched telephone network (PSTN).
An exemplary data-over-cable system with telephony return includes customer premises equipment (e.g., a customer computer), a cable modem, a cable modem termination system, a to cable television network, a public switched telephone network, a telephony remote access concentrator (TRAC 49 in FIG. 1) and a data network 12 (e.g., the Internet). The cable modem termination system 30 and the telephony remote access concentrator 49 together are called a xe2x80x9ctelephony return termination system.xe2x80x9d
In a two-way cable system without telephony return, the customer premises equipment 14 sends data packets to the cable modem 28, which sends the data packets upstream via the cable television network 22 and 26 to the cable modem termination system 30. Such a system is shown in FIG. 1. The cable modem termination system 30 sends the data packets to appropriate hosts on the data network 12. The cable modem termination system 30 sends the response data packets back to the appropriate cable modem 28.
Like all network devices, cable modems filter at the hardware level based on the media access control (MAC) destination address contained in messages received at the cable modem. The MAC layer in the ISO reference model consists of the lower half of the data link layer as defined in the IEEE 802.3 standard. Generally speaking, the MAC layer contains specifications for frame formats as well as the rules for accessing the physical portions of the network. The MAC destination address is matched against addresses stored in the cable modem to determine if the message is intended for the cable modem. These addresses that are stored include the well-known broadcast, multicast and all stations addresses. For cable modems compliant with the DOCSIS specifications, the stored addresses also include the multicast MAC address of the hexadecimal form 01-E0-2F-00-00-01, which enables the addressing of all cable modems on a MAC sublayer domain.
Within this prior art addressing scheme, a message can be sent from a cable modem termination system 30 to a single cable modem (a xe2x80x9cunicast messagexe2x80x9d), and to all the cable modems on a given MAC sublayer domain (i.e., a multicast message directed to all the cable modems in a given MAC domain), but there is no way to address a given subset of cable modems within the MAC domain. For example, there is no way to associate all cable modems on a given upstream channel with a singe multicast address. This would be a useful capability, as it would (1) conserve downstream bandwidth, (2) conserve CPU processing time in the cable modem termination system, and (3) reduce the time for the message to be acted upon, since a single multicast message could be sent quickly to a group of cable modems on an upstream channel, instead of sending a unicast message to each cable modem as would be required by prior art methods. This latter capability of sending a single multicast message to a group of cable modems on an upstream channel rather than sending a series of unicast messages to each cable modem, is critical to systems implementing redundancy or noise avoidance algorithms to facilitate increased reliability, since data links can be lost if timing or nose/power limits are exceeded. However, to the inventors knowledge, the prior art has not provided such a capability.
Thus, in a principal aspect of the invention, improvements in a data-over-cable system are described below which allow for a cable modem termination system or other point in the cable network to send a single multicast message to a group of cable modems on a given upstream channel.
In a first aspect of the invention, an improvement to a cable modem for a data-over-cable system is provided. The cable modem includes, among other things, a machine-readable storage medium, e.g., memory device, containing a set of media access control (MAC) addresses. The improvement comprises including or adding to the set of addresses stored in the cable modem an upstream channel multicast MAC address, with the upstream channel multicast MAC address associated with all cable modems transmitting in a common upstream channel in the data-over-cable system.
The upstream channel multicast MAC address could be autonomously added and updated by the cable modem each time it received a new channel assignment from the cable modem termination system. Alternatively, the upstream channel multicast MAC address could be added via a network management protocol packet such as simple network management protocol (SNMP), or through any other convenient means. The particular upstream channel multicast MAC address added to the set of addresses stored in the cable modem would correlate to the cable modem""s current upstream channel assignment. Upon receipt of a new message from the cable modem termination system, the cable modem would examine the destination MAC address in the header and attempt to match it with its list of stored address. If the destination addressed matches the upstream channel multicast MAC address stored in the cable modem, then the cable modem would process the message. If no match is found, then the cable modem would discard the message.
In this manner, the ability of the cable modem termination system to send a single multicast message targeted to only those cable modems in a particular upstream channel is provided. The cable modem termination system does not have to send unicast messages to send messages to all the modems operating in a given channel. Thus, if the cable modem termination system determines that a problem or impairment is occurring in a particular upstream channel in the cable network, it can quickly and efficiently send messages to the affected modems. This allows redundancy or noise avoidance algorithms to be implemented in efficient manner, while conserving bandwidth on the downstream channel.
In one possible embodiment of the invention, the upstream channel multicast MAC address comprises a modification to one or more bytes in a media access control multicast address. The modification is a replacement of the one or more bytes of the media access control multicast address with one or more bytes identifying the upstream channel that the affected cable modems are in. For purposes of example and not limitation, the media access control multicast address for DOCSIS compliant devices takes the form 01-E0-2F-00-00-01 (hex). This multicast address could be modified to become the inventive upstream channel multicast MAC address by changing the address to either 01-E0-2F-xx-00-01 or 01-E0-2F-00-xx-01, wherein the xx byte comprises two hexadecimal characters that identify the upstream channel of interest.
In another aspect of the invention, an improvement to a cable modem termination system for a data-over-cable system is provided. The cable modem termination system sends messages to cable modems in a downstream channel in the system, with one or more of the cable modems communicating with the cable modem termination system in an upstream channel in the system. The improvement comprises providing the cable modem termination system with the ability or means for broadcasting an upstream channel multicast MAC message to a plurality of cable modems in a downstream channel of the data over cable system. The message identifies an upstream channel in the data-over-cable system and includes an upstream channel multicast MAC address that is stored in at least one of the cable modems in the downstream channel. The only cable modems that respond to the message are those cable modems that store an upstream channel multicast MAC address matching the upstream channel multicast MAC address contained in the message. As in the first aspect of the invention, the upstream channel multicast MAC address may consist of a modification to one or more bytes in a media access control multicast address.
The concept of customizing a multicast MAC message to target a specific group of cable modems could be applied to other types of addressing and identification techniques, and apply to other groups of cable modems besides those communicating in a common upstream channel, in accordance with the invention. For example, a group of cable modems in a data-over-cable network may be dedicated to status monitoring functions, may be assigned to a particular customer, may be located on a particular part of a network, or may be supplied by a certain manufacturer, or otherwise may be grouped together. The cable modem termination system sends a xe2x80x9cgroupxe2x80x9d multicast MAC message, e.g., a status inquiry message, containing a xe2x80x9cgroupxe2x80x9d multicast MAC address, that is to be responded to only by those cable modems that belong to the group. The group multicast MAC message has an address that includes a particular byte or bytes that are associated with the cable modems in the group. This address will have been previously stored by the cable modems that are in the group so that they can respond to the group multicast MAC message. When the multicast message is sent, all the cable modems that receive the message check the address with the addresses stored in memory. If a match is found the cable modem responds. If not, the message is discarded.